According to a known fastening bolt provided with the function of a torque limiter, the bolt comprises a head member provided with a tool engaging head configured to be engaged by a fastening tool, and a male thread member formed with a male thread and coupled to the head member via a ratchet mechanism. The ratchet mechanism firmly joins the head member and the male thread member against a relative rotation when the head member is turned in the unfastening direction. When the head member is turned in the fastening direction with a fastening torque less than a prescribed limit value (maximum fastening torque), the fastening torque is transmitted from the head member to the male thread member via a pair of mutually opposing slopes which are urged toward each other by a spring so that the fastening torque is transmitted from the head member to the male thread member owing to the frictional engagement between the mutually opposing slopes. However, when the head member is turned with a fastening torque greater than the prescribed limit value, the mutually opposing slopes start slipping relative to each other so that the male thread member is prevented from being threaded into the nut with a fastening torque exceeding the prescribed limit value. (See JP2000-27886A, for instance.)
According to a known fastening nut provided with the function of a torque limiter, the nut comprises a head member provided with a tool engaging head configured to be engaged by a fastening tool, and a female thread member formed with a female thread and engaging the head member via mutually opposing thrust surfaces frictionally engaging each other. A spring member interposed between the head member and the female member urges the mutually opposing thrust surfaces toward each other so that the torque in the fastening direction can be transmitted from the head member to the female thread member owing to the frictional engagement between the mutually opposing thrust surfaces, and when the head member is turned with a torque greater than a prescribed limit value, the thrust surfaces slip relative to each other so that the female thread member is prevented from being threaded onto a corresponding male thread with a fastening torque greater than the prescribed limit value. (See JP10-47326A, for instance.)
The conventional bolts and nuts provided with the function of a torque limiter were inevitably large in size because of the structural complexity, and were not configured to allow a large torque limit value to be set without unduly increasing the spring biasing force. Therefore, not only the size but also the difficulty of the assembly and servicing pose a serious problem.